After a short career as a Wall Street stockbroker, Capstick headed to Latin America, where he traveled widely while hunting, and fishing. A few years later he returned to New York, where he founded a business arranging professionally guided hunting trips. Shortly thereafter he took a position as hunting and fishing director of Winchester Adventures of New York.  In that capacity he made his first trip to Africa in 1968. Subsequently he worked as a professional hunter and game ranger in Zambia, Botswana, and Rhodesia.

Capstick started writing about his adventures in the late 1960s, and published numerous articles in various sporting magazines. In 1977 he published his first book, Death in the Long Grass, which became a commercial success and established his reputation as an author of true adventure stories. Capstick is frequently compared to Ernest Hemingway and Robert Ruarak in discussions of influential African hunting authors.